1. Field of the Invention
The present invention relates to an information recording medium and an information recording and reproducing slider and, more particularly, to an information recording medium which is improved in recording/reproducing characteristics according to which information is magnetically recorded and reproduced, as well as to an information recording and reproducing slider suited to recording and reproduction of information on and from the information recording medium.
2. Description of Related Art
Optical recording media such as DVDs and magnetic recording media such as HDDs have been known and have gained popularity as information recording media. In addition, since it has been desired to enhance the recording density of information, research has been made into magneto-optical (MO) recording media in which the merits of optical recording media and the merits of magnetic recording media are combined.
According to an information recording/reproducing method of magnetically detecting information recorded on a magnetic material of the magnetic recording medium or the magneto-optical recording medium among the above-described recording media, as the result of more advanced research, it has become possible to record information at ultrahigh density. It has presently been proved that there is a possibility of ultrahigh-density recording exceeding 10 Gbits/(inch)2.
A polycrystalline magnetic material, such as CO77Cr15P Ta2, which has an easy axis of magnetization in an in-plane direction is normally used as such a magnetic material.
Use of a perpendicular magnetization film made of a rare earth-transition metal amorphous alloy as the magnetic material of the magnetic recording medium is described in Japanese Unexamined Patent Publication No. SHO58(1983)-165306. This rare earth-transition metal amorphous alloy is used as a magnetic material for magneto-optical recording media, and magneto-optical recording media having a laminated body of rare earth-transition metal amorphous alloys having different magnetic characteristics have recently been described in Japanese Unexamined Patent Publication Nos. HE15(1993)-217226, HE15(1993)-325283, SHO63(1988)-302448 and the like.
Among them, Japanese Unexamined Patent Publication No. SHO63(1988)-302448 describes a magneto-optical recording medium having a laminated body in which a so-called rare earth (RE) rich film exhibiting the magnetic moment of rare earth metal larger than that of transition metal is exchange-coupled to a so-called transition metal (TM) rich film exhibiting the magnetic moment of transition metal larger than that of rare earth metal.
More specifically, a description is made of a magneto-optical recording medium in which a TbFeCo film having a larger Kerr rotation angle is used as the TM rich film and a TbFeCo film capable of providing a large signal level (√{square root over (R)}·θ:R=reflectance, θ=Kerr rotating angle) is used as the RE rich film, and both films are exchange-coupled to each other. It is stated that this medium has advantages in that a large SNR can be obtained because a rise in noise level can be restrained, and that writing energy can be made small. In other words, it can be considered that if the medium described in this publication is used as a medium such as a MO disk, recording noise can be reduced while retaining a large magneto-optical effect.
To improve the recording density to a further extent, noise which occurs in an information recording medium needs to be reduced to a further extent. To this end, the grain size of magnetic grains (crystal grains) must be made approximately 10 nm.
However, if the grain size of the crystal grains of the above-described polycrystalline magnetic material is made approximately 10 nm, the resulting magnetic domains (recorded bits) become thermally unstable at, in particular, the interfaces of the crystal grains. This fact causes problems that noise is produced in the information recording medium, recorded information disappears and the like. In particular, the adoption of such minute magnetic grains causes the problem that the coercive force of the information recording medium becomes lower with a temperature rise (normally, the temperature inside a drive in use becomes approximately 65° C.).
In addition, it cannot be said that the medium described in the above-cited Japanese Unexamined Patent Publication No. SHO63(1988)-302448 is appropriate for a method of reproducing magnetic flux. For example, in Example 1 of this publication, a description is made of a medium formed of a laminated body of a magnetic film of Hc=2 kOe (thickness=20 nm) and a magnetic film of Hc=10 kOe (thickness=60 nm). This medium has the coercive force substantially larger than that recordable by a normal magnetic head, and thus the recording is difficult.
In addition, it is in general necessary that the magnetization of a medium be large to an extent detectable by a magnetic head.
In the case of the medium of the above-cited publication, it is inferred that the magnetization of the magnetic film of Hc=2 kOe is comparatively large and the magnetization of the magnetic film of Hc=10 kOe is comparatively small. Total magnetization is the sum of the values of magnetization of magnetic films which constitute a medium. However, in the case of the medium of the above-cited publication, since the film having a larger magnetization is thin, its total magnetization is very small and the medium does not emit magnetic flux to the outside, so that detection by a magnetic head is difficult.